X-ray studies of Active Galactic Nuclei
Poshak Gandhi
European Southern Observatory, Alonso de Cordova 3107, Casilla 19001, Santiago, Chile
1. Introduction
Unlike modern optical astronomy that began with the first use of telescopes about 400 years ago,
the study of the Universe in X-rays is only about four decades old. The pathlength of typical
cosmic X-rays in the Earth’s atmosphere is only a few cm – this opaqueness being due to high
photoelectric absorption cross-section of X-ray photons in air. The very small wavelength of Xrays also necessitates new ways of focusing them in order to form images. These limitations were
overcome only with the advancement of technology. X-rays from the Sun were first discovered during
short rocket flights in the 1940s; but the flight that marked the birth of cosmic X-ray astronomy
was carried out on June 12, 1962. Aiming to search for X-rays from the Moon, this mission ended
up discovering Scorpius X-1 (the brightest point-source in the X-ray sky later recognized to be a
neutron star in a binary system), as well as the X-ray background, a faint isotropic glow of radiation
whose origin remained a mystery for almost 40 years [1]. Tremendous progress has been made since
then, and the latest all sky surveys can now boast more than 100,000 detected X-ray sources.
The X-ray spectral regime covers energies of several hundred keV down to 0.1 keV (equivalent to
wavelengths of less than 0.1 Å to above 100 Å). These energies are associated with thermal temperatures above 107 K, higher than those of stellar photospheres by factors of several thousands or more
(see Fig. 1). Non-thermal processes that generate X-ray photons are: bremsstrahlung; synchrotron;
discrete quantum transitions, e.g., fluorescence, leading to line radiation; and Comptonization. Clusters of galaxies are the largest bound structures in the observable Universe. The presence of ionized
gas within large, deep potential wells provides the right conditions (density, temperature) for the
bremsstrahlung cross-section to be significant. Synchrotron processes are associated with intense
magnetic fields in compact objects. In collimated astrophysical jets, synchrotron radio emission can
be inverse-compton scattered up to X-ray energies by the very electrons that produce the radio
photons, in a process known as Synchrotron Self Compton. The most important process from the
perspective of the sources discussed in this paper, however, is Comptonization.
One of the most important astronomical discoveries of the past decade is the fact that every
large galaxy, including the Milky Way, harbours a nuclear super-massive black hole (SMBH; [2]),
with mass ranging from 106 − 109 solar masses (M¯ ). About 10 per cent of these appear to be
extremely bright active galactic nuclei (AGN) at any given epoch. Typical AGN X-ray luminosities
range from 1042 − 1046 erg s−1 . This should be compared with the bolometric luminosity of the
Sun (L¯ ) ∼ 4 × 1033 erg s−1 . Thus, AGN can easily generate between 109 and 1013 L¯ of power in
X-rays alone, enough to rival an entire galaxy.
AGN were first discovered in the 1940s as point-like sources of powerful optical emission with
spectra showing very broad and strong emission lines having associated doppler velocities of order
several thousand km s−1 . They were also found to show significant optical variability on time-scales
of months, with the emitting source being completely unresolved. Strong X-ray variability is observed
over periods of hours to days, implying that the emission region must be very compact, indeed (since
net, observed variability is governed by the source light-crossing time). The inferred power and
compactness of AGN is most easily explained by accretion of hot gas onto the SMBH and liberation
of the gravitational energy associated with this infalling matter through dissipative processes; this
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link between highly variable sources with strong optical emission and accreting SMBHs was first
established in the late 1960s [3].
AGN have since been studied at every available wavelength from radio to Gamma rays, and
are found to be prodigious sources of emission across the whole electromagnetic spectrum. X-rays,
however, are the most direct probe of the accretion processes responsible for this massive energy
generation because they can penetrate through large columns of obscuring gas and dust associated
with the accreting matter that surround many AGN. Moreover, X-rays are generated in the very
hearts of these sources, down to distances within a few gravitational radii of the central black hole
(BH), while emission at other wavelengths, e.g., optical radiation, is generated at distances 10 5 times
or more farther out.
AGN are the dominant sources of cosmic X-rays. In any medium-deep image of the X-ray sky
at high galactic latitudes, the majority (about 80 per cent) of detections will be AGN, followed by
distant galaxy clusters and X-ray binaries within the Galaxy. This knowledge is relatively new –
until the early 1990s at least, the X-ray sky was observed to be dominated by a diffuse cosmic X-ray
background radiation that could not be resolved into individual sources with the previous generation
of X-ray telescopes; this background is now recognized as the integrated emission from distant AGN
populations. The role of powerful AGN feedback through winds and ionization of the interstellar
media is now seen as an integral part of the process of galaxy formation; AGN can no longer be
ignored as rare and compact singularities with no significant impact on their environments. Some of
the most recent X-ray surveys are revealing unexpected populations of AGN in the distant Universe,
and suggest that there may have been more than one major epoch of black hole mass accretion
assembly in the history of the Universe.
This paper presents a brief summary of the field of observational AGN X-ray studies. Due to
the large scope of the subject, this work is not intended to be comprehensive, but it is an up-to-date
analysis of phenomenological studies of AGN in X-rays. In section 2, we outline the knowledge of
AGN structure and characteristics that has been built up from multi-wavelength observations over
the past few decades. This also includes the standard AGN Unification picture and describes X-ray
spectra of AGN in some detail. X-ray detection technology and important satellite missions that
studied AGN are mentioned in section 3. Finally, the importance of AGN in the process of galaxy
formation and evolution is outlined and key results from the latest generation of X-ray satellites,
Chandra and XMM-Newton, are discussed.
2. AGN structure and fiducial characteristics
Fig. 2 shows the main physical components believed to exist in most AGN. Most physical sizes are
scaled with the radius of the event horizon, which is linearly dependent on mass as R S = 2GM/c2
for a non-spinning BH. For a Solar mass BH, this radius is approximately 3 km. However, stable
structures cannot always extend down to the event horizon. The inner-most stable orbit for matter
in the space-time of a non-spinning (Schwarzschild) BH is 3RS , but for a maximally-spinning, Kerr
BH, both the event horizon and last stable orbit are found at RS /2. Increasingly, evidence is being
found that many black holes may be rapidly spinning [5, 6]: these objects are excellent probes of
regions closest to the BHs themselves.
Studies of X-ray emitting stars within the Galaxy, primarily X-ray binaries, suggest that accreting matter, as it falls inwards, settles in a planar orbit around the central source (an ‘accretion disk’)
where most of the gravitational energy release occurs due to dissipation. The overall efficiency of
energy release in this process is higher than most other processes known 1 , including nuclear fusion,
1 A fiducial value for the efficiency of conversion of gravitational accretion energy into radiation for bright AGN is
10 per cent. The equivalent efficiency of mass:energy conversion for nuclear fusion of Hydrogen is only ∼ 0.7 per cent.
2
Figure 1: View of the sky region covering the Orion constellation compiled from a series of images taken by the
ROSAT X-ray observatory (left), compared to the same region as it would be seen in optical light (right). The
sky appears quite different in the two bands. The brightest source in the optical image is the Moon, which is
conspicuously faint in X-rays. Additionally, the brightest star in visible light is Sirius A, at the left edge of the
image. In X-rays, the star seen at this position is actually Sirius B, a known white dwarf companion to Sirius
A, that is completely invisible in the optical regime (http://wave.xray.mpe.mpg.de/rosat/five years/survey/,
[4]).
and is enough to explain the incredibly high power-outputs of X-ray binaries and SMBHs.
Accretion disk sizes are thought to be of the order of light-days for typical SMBHs of mass 10 6
M¯ . However, even at the distance of the nearest AGN, such sizes are too small to be resolved by the
current generation of telescopes, since the resolution required is close to 1 milli-arcsecond, or about
10−7 degrees (though there are some suggestive claims in this regard; [7]). Improved interferometric
techniques on the largest ground-based telescopes may soon be able to resolve the disks for the
very nearest AGN. There exists one, fortuitous case, NGC 4258, where it has been possible to map
motions in the accretion disk due to presence of a strong water maser [8]. Note that any large,
stable accretion disk is unlikely to exist in our Galaxy, since the central black hole is only weakly,
and sporadically, active [9].
Typical AGN accretion disks are optically-thick and physically-thin [10]. Associated temperatures are in the range of ∼ 105 −106 K, making them sources of quasi-thermal optical and ultraviolet
radiation that scatters off electrons commonly found in the ambient media of hot and ionized accretion regions; this ‘corona’ of electrons is heated by energy feeding from magnetic fields (similar to
the mechanism of heating of the Solar corona). Cool accretion disk photons thus undergo inverseCompton scattering off hot electrons, and emerge as higher energy X-rays. Multiple scatterings
within the corona increase the energy further, resulting in characteristic power-law non-thermal
spectra extending from under 1 keV to several hundred keV. More details of the main features in
AGN X-ray spectra are discussed in section 2.2.
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Discrete and patchy cloud-like structures much further out from the SMBH produce the bulk of
AGN optical emission line radiation, according to which AGN were first classified. Essentially, those
with very broad (full width at half maximum – FWHM – of order 3000 km s−1 or more) permitted
recombination lines also typically have blue broad-band optical continua and are designated ‘type
1’ AGN. Those with comparatively narrow (FWHM ∼ 500 km s−1 ) permitted lines are designated
‘type 2’ AGN and usually possess red continua. Intermediate types also exist which display both
narrow and broad components to the permitted lines, and all types can show significant forbidden
lines, but these are always narrow.
Photoionization is now accepted as the principal mechanism for emission line generation, though
collisional excitation can have a significant effect, especially for forbidden lines. The fact that
forbidden lines are always observed to be narrow suggests the existence of two distinct regions where
optical emission principally occurs. A dense (107 − 1010 cm−3 ) broad line region (BLR) is inferred to
exist on scales much smaller than a parsec (pc) within the deep BH gravitational potential, leading
to the observed large doppler-broadened line-widths. A technique called reverberation mapping that
uses the delay time between continuum variation and subsequent emission line intensity variations
suggests the size of this region to be 0.1L0.5
46 pc (where L46 is the photoionizing luminosity in units
of 1046 erg s−1 ; [11]). The narrow line region (NLR), on the other hand, extends over scales of tens
to hundreds of pc, where the emitted lines are all narrow due to weaker gravity. The very existence
of strong forbidden lines implies that densities in the NLR cannot exceed the critical density for
collisional de-excitation, and are thus inferred to be low (∼ 104 cm−3 ) from analysis of various
nebular line species such as the [Oiii]λλ4959, 5007 doublet. Temperatures of the photoionized
media are difficult to ascertain due to dependence of radiative cooling on metallicity, but values of
∼ 104 K are typical for the NLR; the BLR being hotter by a factor of ∼ 1.5 − 2 [12].
In addition, AGN have been classified according to their luminosities at radio wavelengths (compared to the optical luminosity). Radio-loud AGN were among the first to be catalogued in large
numbers and can have distinctive jet and/or lobe morphologies [13]. Radio-quiet AGN are difficult to identify, especially at high redshift, but probably outnumber their radio-loud counterparts
by a factor of 5 or more. Other classes of AGN include BL Lacertae objects, blazars and LINERs.
For comprehensive discussion of the different AGN classes, consult the following references: [14 - 16].
2.1 The obscuring torus and AGN Unification
The variety of seemingly distinct classifications can be united to a large extent under what has
now come to be known as the AGN Unification scheme. According to this hypothesis, the common
powerhouse in each type of AGN is an accreting SMBH, while most of the differences amongst the
classes can be ascribed to anisotropic AGN obscuring environments and their random orientations
towards Earth (Fig 2). The idea of unification first gained credence through observations of the
type 2 Seyfert2 NGC 1068 [17], in which a broad-line type 1 AGN was uncovered through polarized
light. If optically-thick obscuring matter (mostly dust) blocks the line-of-sight to the nucleus of
NGC 1068, the broad permitted lines (generated close to the nucleus) are not seen directly. But if
other sight-lines are not completely obscured, a small fraction of radiation could be scattered into
our direction. The absorbing matter is thus not sky-covering (as seen by the AGN) and is generically
imagined to be in a ‘torus’ geometry. The polarization of the radiation suggests that the reflector
consists of electrons in a plasma, presumably ionized by direct AGN radiation. The narrow-line
2 AGN which are fainter than M ∼ −21.5 + 5 log(h) [where M is the absolute Vega B-magnitude and h is the
B
B
Hubble constant in units of 100 km s−1 Mpc−1 ] in the optical are traditionally referred to as Seyferts; QSOs and
quasars are sources with higher luminosities. In X-rays, the Seyfert / QSO classification threshold luminosity is at
∼ 1044 erg s−1 .
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region in all AGN lies above the torus and is irradiated by the photoionizing continuum.
In the local Universe, obscured AGN are seen to be more common than unobscured AGN by a
ratio of at least 3–4 [18]. Whether this ratio evolves out to higher redshifts is an important question
currently under study.
Thus, the size and orientation angle of the torus is the key variable that decides the appearance
of an AGN. Broad permitted lines in type 2 AGN have also been discovered in the near-infrared [19],
where the opacity is much lower than at optical wavelengths, while re-processed, warm, thermal radiation from the torus itself has been detected in the far-infrared [20]. It is the X-rays, however, that
have the highest penetrating power through obscuring gas that is associated with the enshrouding
torus dust.
Figure 2: Schematic of an AGN. The central Black Hole accretes infalling matter from the accretion disk whose
characteristic temperature of the order of several hundreds of thousands of Kelvin results in thermal ultraviolet
emission. This radiation is scattered off a hot electron corona above the accretion disk, gaining energy in the
process and emerging as X-rays. This radiation ionizes gas that exists in the form of patchy clouds at distances
of several light-days to light-years from the central source, resulting in characteristic optical broad and narrow
emission lines. Extended powerful jets and ionization cones are also observed in many AGN; note that they are
expected to be symmetric about the SMBH, though in the diagram they are shown on one side only, for clarity.
The dusty torus absorbs radiation along certain lines of sight. If the orientation of the AGN is such that light
reaches Earth directly (without passing through the torus), we would see an unobscured AGN. Otherwise, an
obscured AGN would be observed. If the orientation angle (or, optical depth) is large, this obscuration can
absorb most of the direct AGN light, resulting in sources that are optically-thick even to photons upto 10 keV;
these sources are called Compton-thick, and are dominated by indirect radiation scattered into the line-of-sight.
Diagram is not to scale.
2.2 AGN X-ray spectra
There are many prominent features seen in the X-ray spectra of AGN. Several of these are discussed
below and shown in Figs. 3 & 4. A few important, but relatively infrequent spectral features,
including warm absorbers and soft blackbody excesses, are not discussed here.
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• AGN for which there is a direct line-of-sight to the central black hole are characterized by
power-law continua over the 0.5–10 keV regime. Due to the way in which X-ray detectors
work, observed spectra are usually parametrized as power-laws in terms of photon density:
NE ∝ E −Γ cts s−1 cm−2 keV−1 . In the local Universe, an average value of the photon-index
Γ = 1.9 is observed [21] for AGN, with typical variation over Γ = 1.7 − 2. The power-law nonthermal spectrum arises due to multiple up-scatterings of accretion disk photons undergoing
inverse-Compton scattering off Maxwellian-distributed hot electrons above the accretion disk.
• A broad hump, or excess of power, is seen in many AGN over ∼20–40 keV above the canonical
lower-energy power-law. This can be explained as due to reflection of photons that are scattered
from the corona back towards the accretion disk [22]. The reflection fraction depends on
the ionization state of the disk, but the overall effect is predicted to be a ‘hardening’ or
biasing of the spectrum towards higher energies, due to high albedo at 30 keV combined with
photoelectric absorption of lower energy X-ray photons.
• Above several tens to hundreds of keV, X-ray and gamma-ray telescopes have discovered an
exponential cut-off to the AGN power-law extrapolated from lower energies; the characteristic
cut-off energy can be as high as a few hundred keV. At these high energies (approaching
the electron rest-mass energy of 512 keV), photons lose more energy than they gain in each
Compton scattering because of electron recoil, resulting in a roll-over of the spectrum.
• The generic optical segregation of AGN into type 1 and type 2 classes has an X-ray analogue
as well. The lower energy end of the spectrum in AGN (below 10 keV) is modified by obscuring
gas through photoelectric absorption if the line-of-sight passes through the obscuring torus.
A typical integrated line-of-sight obscuring column density that delineates the boundary between obscured and unobscured AGN is 1022 cm−2 . The absorbing column-density itself is
parametrized in terms of Hydrogen atoms per cm2 integrated along the line-of-sight, though
heavy elements such as O, Mg, Si, S and Fe are responsible for much of the opacity above
1 keV, and discreet photoelectric edges are observed at the ionization energies for these ions
[23]. The obscuration associated with an e-folding decrease of transmitted radiation at 10 keV
in the source rest-frame is ∼ 1.5 × 1024 cm−2 , and these sources are called ‘Compton-thick’.
Very little direct radiation is seen from these sources.
Additionally, at very high column-densities, multiple scatterings, as modelled by the KleinNishina cross-section, lead to a Compton down-scattering over the full X-ray spectral regime,
and very little direct flux emerges above column-densities larger than 10 25 cm−2 [24].
• Discreet quantum transitions in the X-ray regime produce emission lines, primarily due to fluorescence. These had been beyond the reach of study by X-ray detectors until a few years ago,
due to the relatively-high resolving power required to distinguish them. Iron transitions are
the most common lines found in AGN X-ray spectra, due to a combination of high abundance
and fluorescence yield, though the recent use of gratings capable of dispersing X-rays into their
component energies has revealed an abundance of other elements [25].
Emission lines are very sensitive probes of the inner regions of the accretion disks, due to
several relativistic effects that affect their observed profile. The rotation of the accretion disk
leads to the doppler-splitting of the line on the approaching and receding sides of the disk
relative to the observer; line photons lose energy due to strong gravitational redshift as the
photons climb out of the deep potential well associated with the central BH; further distortion
occurs due to relativistic boosting of the blue (approaching) part of the spectrum relative to
the red part and due to the transverse doppler effect associated with time dilation. The net,
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predicted line profile has a blue peak and a hump with an extended red tail (see Fig. 4), each
feature being sensitive to BH characteristics such as mass, spin etc. Such a line profile has
been observed in several local [26] as well as very distant [27, 28] AGN, thus also providing a
laboratory to test these relativistic effects in action.
Figure 3: AGN rest-frame X-ray spectral energy distributions (SEDs) showing key spectral features discussed
in section 2. The spectra were simulated using the X-ray spectral analysis code xspec [29]. The y-axis units
are such that a horizontal line denotes equal power per unit decade of energy. The intrinsic AGN power-law
is modelled with photon-index Γ = 1.9 (the equivalent exponential index in the units of the above plot is
Γ + 2). The spectra include a reflection hump from cold material at solar abundance assuming uniform 2π
reflection and an inclination angle of 60 degrees, as well as an exponential cut-off at 360 keV. Obscuration
by material of solar abundance local to the source is modelled [with log(NH ) = 20.25 to 25.25, labeled] using
the cross-sections of Morrison & McCammon [23], and Compton down-scattering is modelled with the full
Klein-Nishina cross-section. Finally, 2 per cent of the primary radiation is added to each SED to model the
scattered component into the line-of-sight. See Gandhi & Fabian [30] and Wilman & Fabian [24] for details.
The shaded region marks the X-ray spectral regime that is accessible to study to current high spatial resolution
X-ray instruments such as Chandra and XMM-Newton.
Thus, characteristic AGN spectral features are generated on a wide spatial scale in the nuclear
surroundings, with the X-ray view penetrating closest in to the black hole itself. AGN energy output
is considerable across the entire spectrum, and is the result of various physical processes occurring
on differing scales. The Unification scheme has provided a feasible frame-work for progress in our
understanding of AGN structure and environment. Recent observations suggest, however, that the
true nature of AGN may be different from that implied by this Unification scheme. Before discussing
these, we present a brief review of AGN X-ray detection technology and surveys carried out so far.
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Figure 4: (Left) AGN Fe line profile distortions due to dynamical and relativistic effects discussed in the
text and labelled on the plot. The overall line profile is shown in the bottom plot, as a function of the ratio
of the observed frequency to the frequency at which the line photons were emitted. (Right) Observed 6.4
keV Fe line in the Seyfert galaxy MCG–6-30-15, showing the best such data available to date. The data are
compiled from ∼ 300 kiloseconds of XMM-Newton exposure time, and provide spectacular confirmation of
the aforementioned predicted effects. Incidentally, the extended red wing of the line suggests that line photons
undergo large gravitational redshifts that naturally arise from the deep potential well of a rapidly spinning
black hole Credit: A.C. Fabian [26].
3. X-ray Observatories and AGN Surveys
After brief rocket flights that carried proportional counters into space in the 1960s, the first, dedicated
X-ray satellite to carry out a uniform all-sky X-ray survey was Uhuru, launched in 1970 [31]. It
discovered a total of 339 sources to a limiting sensitivity of ∼ 10−3 times the flux of the Crab Nebula,
most of these being X-ray binary stars and supernovae remnants, but also some Seyfert Galaxies
and galaxy clusters. Uhuru is also credited with discovering the diffuse X-ray emission associated
with clusters of galaxies.
Towards the end of the 1970s, NASA’s High Energy Astronomy Observatory (HEAO) series of
missions were a milestone for X-ray astronomy. With scintillation detectors in addition to proportional counters, HEAO provided complete surveys of the X-ray sky, especially high galactic latitude
regions [32], and good characterization of the diffuse X-ray background radiation over the energy
range 3–50 keV [33]. Incidentally, Aryabhata, India’s first indigenous satellite, launched in 1975,
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also carried an X-ray detector.
None of these missions had the capability of focusing X-rays to form sharp images. Scintillation
detectors and proportional counters can only measure the energy of an incoming photon, and there
is no spatial resolution possible within their field-of-view. The Einstein satellite was the first X-ray
telescope with focusing optics necessary to produce images (see section 3.1 below for a thorough
discussion of X-ray focusing optics). It was the second satellite in the HEAO series, operational during the 1980s, and imaged thousands of extragalactic X-ray sources, as well as diffuse and extended
sources over 0.2–4 keV with superb spatial resolution of only a few arcseconds. It also carried a
proportional counter sensitive up to 20 keV. Other key missions of the 1980s included the European
Space Agency’s EXOSAT and the Japanese Ginga satellites.
The ROSAT, or Röentgen, satellite was the major German/US/UK collaborative X-ray observatory over the soft (< 2 keV) energy range. It was used to carry out several surveys of varying
depth, the largest of which was an all-sky survey containing more than 100,000 detections in all
(See Voges et al. [34] and references therein). With its large collecting area and high resolution optics, ROSAT was able to completely resolve the soft X-ray background into discrete sources (mostly
AGN) through ultra-deep observations [35]; the origin of this background had remained a mystery
for almost 40 years.
ASCA [36] and BeppoSAX were important Japanese- and Italian-led missions of the nineties
respectively, that were responsible for detailed studies of AGN continua as well as Fe emission lines.
BeppoSAX was sensitive over a very large energy range from 0.1–300 keV. With a point-spreadfunction of approximately 1 arcminute, it was also able to rapidly localize Gamma Ray Bursts over
this spatial scale [37].
As mentioned, X-ray sources are expected to be highly variable due to their compactness. This
flux variability has been studied in detail by the dedicated satellite Rossi X-ray Timing Explorer
(RXTE), which continues to operate today. RXTE is sensitive over the range 0.2–250 keV, and has
made several important discoveries, including variable kiloHertz Quasi-Periodic Objects, and confirmation of the fact that AGN Fe fluorescence line flux variations need not correlate with continuum
variability [38].
3.1 X-ray imaging technology
X-ray detectors, for many years, were relatively simple photon counting devices without much spatial
resolution or ability to focus X-rays. Proportional counters are like photon buckets that produce an
ionization charge dependent on the energy of the incoming photons. Some spatial resolution may be
obtained by the use of coded aperture masks, which are sheets of opaque material placed in the optical
path with a well-defined pattern of pinholes to let photons through. Source image construction
is achieved by deconvolving the pattern of the pinholes from the observed pattern projected by
incoming photons. Though this allows only limited spatial resolution, the total collecting area for
even high-energy (hard) X-ray photons can cheaply be made very large.
X-ray focusing instruments had to await advances in technology that allowed polishing of surfaces
to extremely high precision. A typical X-ray photon has an energy higher than an optical photon by
a factor of a 1000 or more. Normal incidence mirrors that are used to focus visible light completely
absorb X-rays falling directly on them, making it impossible to use well-known telescope techniques
in the optical regime. However, just as a solid piece of rock can bounce off the surface of water if
deflected at a grazing angle, X-rays can be focused using grazing incidence reflection off surfaces
that are almost parallel to their incoming direction (Fig. 5).
There are two basic requirements needed to put such a system into practice. The first is that
the angle of reflection be very small with respect to the surface. It can be shown that this angle
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√
varies as ρ/E, where ρ is the density of the reflecting surface, and E is the energy of the X-ray
photon [39]. Thus, the higher the energy of the incoming ray, the smaller has to be the angle of
reflection. For photons of a few keV, this angle turns out to be smaller than 1 ◦ .
The second requirement is that the reflecting surface be very highly polished. Given the minute
wavelength of X-ray photons, and the small angles under consideration, the reflecting surface of the
main mirrors of the Chandra X-ray observatory, for example, are polished to an average surface
roughness of only a few angstroms.
X-ray mirrors are thus highly polished surfaces placed in the path of, and parallel to, the
incoming beam. It turns out that reflection off a paraboloid section followed by a confocal and
coaxial hyberboloid surface provides good focus on-axis as well as at small off-axis angles (current
largest off-axis angles at which reasonable focus is achieved are about a few arcmin). Mirror reflecting
area can be increased by nesting surfaces of different radii within each other. Such a configuration
was first suggested by Wolter [40], and is the current standard of X-ray imaging instruments, such
as Chandra and XMM-Newton, described in the next section.
X-ray detector technology has also progressed steadily. Current solid-state arrays capture individual counts in various ‘channels’, depending on the incoming photon energy. These channels
are like photon-counting devices, and the mapping of channels to the associated photon energy is a
non-trivial (and non-unique) problem. This does, however, allow the simultaneous measurement of
a photon’s spatial position and energy, without the use of dispersing elements. This is only possible
if the flux of X-rays from a source is small enough that only a single photon hits any given detector
element between two consecutive read-outs, a condition easily satisfied for most studies of distant
AGN with the current generation of observatories.
Figure 5: The principal components of a Wolter type 1 nested X-ray focusing mirror configuration. The
double reflection off two surfaces with different curvatures (paraboloid and hyperboloid) is done in order to
ensure good focus over extended regions in the focal plane, while the nesting of mirrors is done in order to
increase the overall effective area. Alignment, polishing and coating of the mirror surfaces are the most critical
parts of the assembly. Image courtesy of http://chandra.harvard.edu/.
3.2 Chandra and XMM-Newton
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The Chandra mission is one of the key missions of NASA’s Great Observatories Origins Program. It
has four polished and iridium-coated mirror surfaces with roughness close to only a few angstroms. It
is named after the Nobel-prize winning astronomer, S. Chandrasekhar, who is credited with the first
detailed calculations of black hole critical mass limits. The result is excellent spatial resolution of
approximately half an arcsecond – the best resolution of any X-ray instrument yet built or conceived
for up to the next 10–20 years. Whereas most previous X-ray surveys required detailed multiwavelength follow-up of sources simply to localize and identify them, the sub-arcsecond resolution is
good enough for direct counterpart-matching and source identification in a majority of point-source
observations.
The combination of high sensitivity over the 0.5–8 keV range and excellent optics implies that
very faint sources can be detected with Chandra. One of the main results from the mission has been
the resolution of the hard cosmic X-ray background radiation (CXB) up to approximately 8 keV
[41]. It is now recognized that the bulk of the CXB is the summed emission from a distribution of
redshifted, and predominantly-obscured AGN. The exact nature and distribution of these distant,
faint AGN are now under intense scrutiny [42]. Two sets of X-ray transmission gratings that are
present in the optical path of Chandra have been used to observe large numbers of emission lines
from X-ray sources, including AGN. For instance, the spectrum obtained for the AGN at the centre
of NGC 3783 shows hundreds of absorption features due to various ionic species of Fe, Mg, O, N,
Ne, Si etc. [25].
XMM-Newton is ESA’s major currently-operational X-ray observatory; it complements Chandra by having a much larger collecting area (a factor of ∼10 larger at 2 keV) over 0.3–10 keV, but
a worse spatial resolution by a factor of a few. The mirror surfaces are coated with Gold. Due to
its large collecting area and good spectral resolution, XMM-Newton is able to search for spectral
features in distant cosmic sources fainter than those detectable by any previous mission: e.g., Fe
emission lines in distant AGN [43] and emission from the cores of distant galaxy clusters (cooling
flows; [44]).
3.3 Future X-ray missions
Astrosat3 is the latest of India’s indigenous space observatory projects, due to be launched in 2007
into a near-Earth, equatorial orbit by the Polar Satellite Launch Vehicle [45]. It is designed as a
multi-wavelength observatory for spectral, imaging and timing studies of cosmic sources, including
AGN. Four payload instruments are planned and will cover the UV(1000-3000 A), soft and hard
X-ray regimes (0.3 - 8 keV; 2 - 80 keV).
On a longer (10–20 year) timescale, ESA’s planned XEUS mission will have an effective area
larger than that of Chandra by a factor of about a thousand, while NASA’s Constellation-X mission
will be a collection of small synchronous satellites with mirrors, acting as interferometers to achieve
the combination of very high collection area and good resolution. It must be realized that the peak
effective area of the Chandra imaging instrument ACIS is equivalent to that of a telescope with a
diameter of only 30 cm (assuming normal incidence optics). With the next generation of missions,
X-ray astronomy will truly enter the realm of large telescopes such as those currently available at
optical or near-infrared wavelengths. An overview of most past, present as well as planned X-ray
observatories can be found on web-sites related to the Goddard Space Flight Centre. 4
3 http://www.rri.res.in/astrosat/
4 e.g.:
http://heasarc.gsfc.nasa.gov/docs/observatories.html
11
4. Universal accretion in a broad context
4.1 The cosmic X-ray background radiation
The cosmic X-ray background (CXB) was discovered two years before the better-known microwave
background [1]. It covers a very wide energy range, extending from ∼0.1 keV to several hundred
keV (Fig. 6). Above ∼3 keV, the CXB is isotropic to within a few per cent on large scales, after
account is taken of a weak Galactic component as well as the dipole radiation field due to the motion
of our Galaxy. This is strong evidence that the majority of this background must be cosmological
in origin. However, there was no concrete and satisfactory explanation for its existence for almost
30 years.
Different processes are now known to dominate the CXB in different energy regimes and have
been identified at different times over the past 40 years. A distinction is usually made between the
soft CXB below ∼3 keV and the background above this energy. At the softest energies ( <
∼ 0.25 keV),
more than 90 per cent of the CXB probably originates as thermal emission in a local bubble of hot,
optically-thin (∼106 K) gas in the solar neighbourhood.
The Cosmic Energy Density Spectrum
CMB
Flux density on sky [nW/m2/sr]
100
CIB
COB
1
CXB
0.01
1.0E+10
1.0E+15
1.0E+20
1.0E+25
Frequency [Hz]
Figure 6: Observed spectrum of Cosmic Backgrounds from microwaves (CMB), infrared (CIB), optical (COB),
X-rays (CXB) out to gamma-rays and cosmic ray particle backgrounds. The units of the plotted flux density
(nW m−2 steradian−1 ) are such that a horizontal line represents equal contributions of power per unit decade
of frequency. As discussed in the text, the CXB arises from the integrated contribution of AGN with different
luminosities and intrinsic obscuring column densities spread over redshift. The CMB is the afterglow of the big
bang, redshifted from the surface of last scattering, or recombination, while the optical and infrared backgrounds
are due to the summed contributions of stellar and accretion processes. Credit: G. Hasinger [46].
Studies by HEAO-1 found that the spectrum of the CXB over the range 3–50 keV can be fitted
to a diffuse 40 keV thermal bremsstrahlung model [33]. Over the range 3–10 keV, a hard spectrum
was found with a photon-index Γ = 1.4 [N (E) ∝ E −Γ ]. The good fit to the free-free emission model
12
prompted the development of a number of theories in which a diffuse hot inter-galactic medium
(IGM) was the principal emitter, e.g., [47]. However, the total IGM matter and energy density
predicted by such models was found to over-predict the observed density significantly. The nearperfect black body spectrum of the CMB found by the COBE satellite [48] conclusively showed that
there were no Compton up-scattering features of microwave photons due to such an IGM, implying
that discrete sources, and not diffuse matter, must be responsible for the bulk of the background.
The difficulty faced by early CXB synthesis models was that no class of sources was known to
have a spectral slope that either matched the CXB slope above 3 keV, or that could be made to
match the slope by a summation of objects widely spread in redshift. The known sources of powerful
extragalactic X-ray emission included thermal bremsstrahlung cooling in galaxy clusters, massive
X-ray binaries in starburst galaxies and active galactic nuclei (AGN). While AGN were subsequently
shown to constitute the majority of the CXB flux at ∼1–2 keV, most objects known twenty years
ago were bright, unobscured AGN with spectra that were too steep (i.e., soft) and inconsistent with
the CXB photon-index of 1.4 above 3 keV.
Either an entirely new class of AGN (such as AGN with radiatively-inefficient flows 5 ) had to
exist, or a larger population of AGN (with intrinsic, steep spectra similar to those observed, but
flattened by the effects of photoelectric absorption local to the source) had been missed from previous
surveys [50]. This latter, ‘absorbed AGN’ hypothesis now has been shown to be correct, but it had
to await for more than a decade for observational verification.
Operations by the Chandra and XMM-Newton satellites over the past five years have proven
beyond all doubt that the integrated emission from a wide distribution of AGN spread out in redshift
and with different obscuring column-densities can account for the bulk of the X-ray background.
While uncertainty up to ∼ 30 per cent remains on the exact normalization of the X-ray background
intensity due to cross-calibration difficulties between various satellite missions, at least 75 per cent
of the hard X-ray background between 2–10 keV is now resolved [41], but a significant percentage
still remains unidentified if one considers only the high end of this energy range [51]; this could arise
from truly diffuse emission or be due to distant and unresolved obscured / Compton-thick AGN.
Detailed multi-wavelength follow-up of the X-ray sources detected and resolved so-far has revealed several interesting properties, including:
• the discovery of large numbers of so-called type-2 QSOs, or AGN with intrinsic luminosities
similar to those of local quasars, but hidden behind obscuring columns of log(N H ) = 22 or
higher [27, 52]. These are natural analogues of the lower-luminosity Seyfert-2s that have
been known for a long time. QSO2s are being found in large numbers only now because of
the fact that X-rays are more efficient at locating them than optical surveys, and an empirical
observation that the fraction of sources that are obscured may actually decrease as a function of
intrinsic luminosity, making QSO2s less abundant than Seyfert-2s relative to their unobscured
counterparts [53].
• the discovery of large numbers of objects whose powerful X-ray emission suggests the presence
of actively accreting supermassive black holes (i.e, AGN) within, yet which show few or no
signs of AGN activity in visible light [54, 55]. The optical as well as near-IR spectra of these
X-ray bright, optically-normal galaxies (XBONGs; also referred to as optically-weak AGN,
elusive AGN etc.) possess absorption features characteristic of early-type host galaxies and a
common feature is the distinct lack of strong, redshifted emission lines associated with powerful
5 Examples of radiatively-inefficient flows include Advection-Dominated Accretion Flows, or ADAFs; in these
sources, almost all of the accreted energy is lost into the black hole instead of being radiated away, due to ineffective coupling amongst viscous dissipative phases in the accreting matter [49]. These are now known not to be
major contributors to the CXB.
13
(unobscured, as well as obscured) nuclear activity. XBONGs are found in luminous, as well as
faint, X-ray sources, and are thought to constitute ∼10 per cent of all X-ray selected AGN [56],
though the true fraction is likely to be much higher, given the difficulty of classification and
incompleteness of current spectroscopic follow-up [43]. This incompleteness fraction is often
as high as 40 per cent, simply because the targets are too faint to study even with the largest,
current 10 m class telescopes;
• the discovery of a relatively low-redshift peak in the distribution of sources that contribute
to the CXB. Relative to optical quasars and their radio analogues that have been mapped
extensively and are seen to peak in number density at z ∼ 2, the newly discovered X-ray
AGN peak at z ∼ 0.7, even after account is taken of the surveys’ incompleteness. This fact
suggests a resurgence in the phase of Universal accretion activity, possibly associated with
anti-hierarchical growth in which lower-luminosity (X-ray) Seyferts were assembled after their
more powerful (optical) quasar counterparts [57].
Concerning the discovery of XBONGs (second point in the above list), Moran et al. [58] have
pointed out that ground-based follow-up of distant AGN with finite-width spectroscopic slits is
significantly biased by the host galaxy continuum, thus making nuclear emission lines difficult to
distinguish. Diluting AGN light becomes increasingly difficult with increasing nuclear luminosity,
however, unless these objects are completely obscured (4π covering fraction of obscuring dust with
little or no scattering into our line-of-sight) on scales of less than a kiloparsec [27]. The mechanism
which generates such large sky covering factors of obscuration and prevents its dissipation into a
planar disk is unknown. The power of embedded starbursts as well as nuclear radiation pressure could
provide the necessary randomized motions required [59, 60]. XBONGs thus provide an important
tool to study the obscuring environment of AGN; their very existence has important consequences for
orientation-based Unification models, since XBONGs should, instead, appear spherically-symmetric.
Other models considered for XBONGs include sources with extremely large dust:gas ratios (atypical for local Seyferts), low-luminosity AGN and BL-Lacs [56]. Models with centrally-truncated accretion disks undergoing advective accretion have also been considered [61]. The general consensus
is that the more powerful sources (possibly including some obscured quasars; [27]) are likely to be
completely obscured AGN.
4.2 AGN evolutionary schemes
The study of ever-fainter populations of active galaxies seems to be revealing more variety in AGN
activity and making the overall picture more complex. On the other hand, the fact that AGN come
with disparate properties is a natural consequence of the different stages of their life-cycle at which
they are caught, and can be understood as part of an evolutionary scheme. This is an active field
of research at the moment, with results being drawn together from right across the electromagnetic
spectrum.
A significant unknown in this whole puzzle is the mechanism by which this life-cycle begins.
While stellar BHs have a theoretically sound basis of formation from the collapse of an evolved,
massive star, it is not known whether super-massive black holes (that can easily be a factor of 10 9
times larger than stars), form via monolithic, run-away collapse of material onto a much smaller
(stellar?) seed BH, or by the hierarchical process of merging of smaller-component, intermediatemass BHs. There are recent claims of discovery of intermediate mass BHs weighing ∼ 10 3 − 104 M¯
at the cores of globular clusters, but their true nature is still under debate [62].
The fact that bright AGN have been observed at z ∼ 6 [63], less than 1 Gyr after the big bang,
implies that BH formation must be a very efficient process, at least in the early Universe. Strong
14
correlations between properties of SMBHs in the local Universe and the galaxies that host them
(such as the black hole mass vs. brightness or velocity dispersion of the stellar bulge) suggests a
process of symbiotic growth between the two components. The galaxy spheroid feeds the SMBH
with gas and dust from stars; at the same time, the galaxy itself is forming around (and being shaped
by) the central gravitational potential well due to the SMBH. Super-massive black hole growth can
continue until all fuel in the environment is consumed. On the other hand, this growth may be
a self-regulated process: accretion onto the central nucleus is likely to result in some feedback in
the form of outward radiation pressure or a strong wind, that could expel the fueling supply of
gas from the environment, automatically halting AGN activity as well as stellar formation near
the nucleus6 . Phenomenological models incorporating such processes have been elaborated upon by
several authors [64 - 66], and are able to reproduce the observed correlations between black hole
mass and bulge luminosity [67] or bulge velocity dispersion [68, 69]. If the power of the wind is
substantial, there could be a large component of the energy output of AGN that is ‘invisible’ across
the radiated spectrum, since that energy is instead removed mechanically.
Clues about the formation environment and evolution of AGN can also be drawn from their
spatial clustering statistics [70]. From their observed angular (sky-projected) clustering, evidence
has emerged that while X-ray and sub-mm detected sources trace the same large scale structure,
they do not necessarily coincide [71]. This is consistent with proposed evolutionary scenarios in
which peak AGN (X-ray) activity is delayed from the initial peak of stellar (sub-mm) activity while
the black hole is still growing [72], thus providing a natural explanation for the failure to detect
many sub-mm selected sources with Chandra, and vice versa [73].
On the other hand, evidence for a strong correlation between X-ray detections and far-infrared
sources has recently been emerging, and telling us in more detail about type 2 AGN populations.
Recall that obscured AGN are likely to outnumber their unobscured counterparts by a large factor,
and it is likely that the number of Compton-thick AGN has been severely underestimated, simply
because of the difficulty of identifying them. They are expected to be bright in the IR because
of the primary radiation component being absorbed and thermalized by the torus. From deep
observations of two well-studied regions of the sky (referred to as the Lockman Hole and the Hubble
Deep Field North), Fadda et al. [74] found that (while the fraction of mid-infrared sources with
X-ray counterparts is more than 10 per cent) at least 30 per cent of X-ray sources have mid-infrared
(MIR) counterparts discovered by ISO (the Infrared Space Observatory), and this fraction increases
to 63 per cent for the sources detected in the higher energy 5–10 keV band, where the optical depth
through obscuring gas is smaller.
Moreover, the luminosity function (LF) of infrared galaxies observed by missions such as the
InfraRed Astronomy Satellite (IRAS) and ISO has been recently inferred to evolve steeply to z ∼ 0.8
and flatten thereafter [75]. While this IR wavelength regime is dominated by obscured star-formation,
reprocessed AGN accretion activity may also have a non-negligible contribution. Radiative transfer
modelling of the broad-band spectral energy distribution (SED) of a number of ISO sources has
suggested that absorbed AGN emission can indeed account for observed far-infrared SED peaks [20,
76]. This evidence suggests that X-ray type 2 Seyferts and quasars may be ubiquitously detected in
the 10–100 m regime.
Indeed, Gandhi & Fabian [30] were able to construct a synthesis model for the CXB and X-ray
number counts, based on the assumption that type 2 obscured AGN are traced by a LF which follows
6 An important scaling quantity as regards radiation feedback is the Eddington accretion rate, that defines the
maximum rate at which steady, spherically-symmetric matter can be accreted onto a compact source without being
blown away. This can be computed from the balance between radiation pressure and gravity, and scales directly
with the BH mass. The associated radiative luminosity is the Eddington Luminosity and is ∼ 10 44 erg s−1 for
MBH = 106 M¯ .
15
the infrared distribution. The authors found that all observational constraints, including the X-ray
background spectrum, deep source counts and the low-redshift peak of the source distribution could
be reproduced if some number density evolution was allowed in the LF. They predicted that, with
the relatively sharp peak of evolution to z ∼ 0.7, Fe emission lines are likely to leave an imprint on
the integrated spectrum of the CXB, of the order of a few per cent.
Other more recent determinations of the AGN X-ray LF and its evolution also require density
evolution (e.g., Ueda et al. [53] suggest a LF in which the density of sources at any epoch depends
also on luminosity), but with a single evolutionary scheme for both obscured and unobscured AGN.
The detection of obscured AGN in the far-infrared is, of course, still expected, and is starting to be
studied in detail with the successful launch of Spitzer [77], the last of NASA’s satellites in the Great
Origins programme. Spitzer provides excellent coverage of the 3–24 m regime, as well as coverage
at 70 and 160 m. Much more light is expected to be shed on the issue of obscured AGN over the
coming years [78].
5. Summary
X-ray emission is the most general characteristic property of AGN activity. X-rays are generated
within, and give us information about, the very core of the accretion region, and thus probe very
interesting regions of strong gravity and relativistic physics.
BH hole formation and AGN influence is now recognized as an integral part of the process of
galaxy formation. While there now exist a plethora of subclasses of AGN types, it is possible that
these simply depict the various stages of BH growth and AGN activity. An AGN is proposed to
be born in an optically-thick environment, feeding off matter in its surrounding. In this stage, the
accreting matter may obscure all lines-of-sight to the BH. Growth continues until the BH is large
enough to generate a wind or jet powerful enough to punch holes in the surrounding obscuring
matter or to blow it away entirely, essentially cutting off its own fuel supply. The BH would then
emerge from its enshrouding veil and be seen as a typical type 1 AGN. The time that this stage
lasts would depend on the amount of fuel in the AGN environment and its effectiveness to lose any
residual angular momentum and feed the BH before being ejected by AGN feedback. Once all fuel
is completely exhausted, this scenario envisages that the BH would become quiescent like the dead
SMBHs seen at the centres of galaxies in the local Universe, with only occasional flaring due to
episodic accretion or mergers.
From a cosmological perspective, the study of various AGN classes has revealed that a significant
fraction of the Cosmic Infrared background can be attributed to AGN UV and X-ray emission that
is absorbed by obscuring matter and thermalized to longer wavelengths. Studies of the shape of the
X-ray background radiation were the first clue to the presence of large populations of obscured type
2 AGN that are being revealed in the current generation of deep X-ray surveys.
X-ray astronomy is a fledgling science that had to overcome tough technical challenges to advance, but is now rapidly catching up with observations in other, more traditional regimes of the
electromagnetic spectrum. Many more developments can be expected in the near future.
Acknowledgment: The author acknowledges the European Southern Observatory Fellowship Programme through which he is supported, Dr. S. K. Saha for the invitation to participate in this
publication and H. Horst for correcting some errors in the manuscript.
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